* Opinium/Guardian poll finds Biden ahead by 57-40 margin * Biden leads on healthcare, the economy and race relationsDemocratic presidential nominee Joe Biden’s lead over Donald Trump has surged to a record 17 points as the US election enters its final sprint, an Opinium Research and Guardian opinion poll shows.Some 57% of likely voters intend to vote for Biden, while just 40% say they will vote for the incumbent president, the survey shows.The 17-point gap is even bigger than than 57%-41% margin found by CNN earlier this month. It is just short of the lead in the popular vote that Ronald Reagan enjoyed in his second landslide victory in 1984. Four years later, Democratic nominee Michael Dukakis led George HW Bush by 17 points only to suffer defeat, but that poll was taken in July so Bush had ample time to recover.With election day just three weeks away and millions of votes already cast, some Republicans fear a rout in the races for the presidency, Senate and House of Representatives. Ed Rollins, who advises a pro-Trump super political action committee, told the New York Times: “The president’s political environment is terrible. It’s an uphill battle.”Asked by the Times if Trump can still turn things around, Rollins replied: “It’s cooked.”Opinium’s findings for the Guardian suggest that a hectic month that saw the death of the supreme court justice Ruth Bader Ginsburg, Trump’s disastrous debate performance and a White House outbreak of coronavirus that infected the president himself swung the pendulum decisively in Democrats’ favour.Biden has gained five percentage points among undecided voters since September. Democrats also injected momentum into existing supporters, with voters for Biden now more likely to turn out, up from 75% in September to 82% this month.The former vice-president now leads on healthcare, race relations, jobs and even the economy (45% to 43%), usually seen as Trump’s signature issue. His reputation as a successful businessman took a hit from a New York Times investigation into his tax affairs.The research also exposes some key differences from the 2016 election when Trump edged out Hillary Clinton in the electoral college.Both Trump and Clinton were historically unpopular. The president again has a negative approval rating of -11%, with two in five (42%) strongly disapproving of how he is handling the presidency. But this time Biden has a strong positive approval rating of +18%. More than half (52%) of voters approve of his handling of his campaign.Clinton also fared poorly on sexism-charged questions of “likability” and which candidate would voters rather go for a beer with. But in 2020 voters say Biden is more likable than Trump by a 57% to 32% margin.And whereas Trump’s “Crooked Hillary” label and allegations seemed to stick, his attempts to portray Biden as mentally unstable appear to be falling short. In fact, voters say Biden, 77, has better mental stamina than 74-year-old Trump by a 48% to 44% margin.Opinium surveyed 2,003 US adults aged 18 or over from 8 to 12 October. Interviews were conducted online and sampled and weighted to match the demographics of the US adult population as well as factoring in education level and past vote in recent elections.Trump’s core support is notoriously loyal, and still turning out at his resurrected campaign rallies, but there is evidence of some Americans turning against him, even in battleground states such as Michigan, Pennsylvania and Wisconsin.Nearly two in three (62%) of ex-Trump voters (who voted for him in 2016 but will not do so this year) say his handling of the coronavirus pandemic is the reason they switched their vote. In addition, almost half (47%) of ex-Trump voters say his personality and behaviour contributed to the switch.Democrats have said a massive victory is the surest way to avoid lengthy legal disputes that could even spill over into street violence. Trump has spent months seeking to undermine the credibility of the election in general and mail-in voting in particular.Opinium found that Biden’s lead relies on the success of mail-in voting, likely to hit record levels during the pandemic. Some 55% of in-person voters intend to vote for Trump while 42% intend to vote for Biden. But when it comes to mail-in voters, 75% intend to vote for Biden and only 22% intend to vote for Trump.As a result, America may witness a so-called “red mirage” in which Trump appears to be winning based on the early count of in-person votes, only to be overtaken by Biden’s mail-in ballots hours or day later. Only 30% of voters expect to know who the winner is on election night.There are fears that Trump will use that time to spread conspiracy theories and declare victory. Half (50%) of voters are worried that if the president loses the election, he will not concede. There is a partisan divide: two-thirds (66%) of Trump voters are worried that the election will be rigged.In the meantime, Republicans are racing to confirm Amy Coney Barrett to the supreme court before election day. More than half (55%) of Americans think the court would become skewed towards a conservative viewpoint if Barrett joined it. A third (32%) think it will become “very conservative”.Subsequently, two in five (41%) think the new court would vote to overturn Roe v Wade, the landmark ruling that legalised abortion nationwide. This is despite a plurality of support for the ruling (45%).